1. Field of the Invention
The present invention generally relates to quantum dot nanoparticles. More particularly, it relates to the synthesis of quantum dot nanoparticles containing no cadmium or other heavy metals.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
There has been substantial interest in the preparation and characterization of compound semiconductor particles with dimensions in the order of 2-100 nm, often referred to as quantum dots (QDs) and/or nanocrystals. This interest is mainly due to their size-tunable electronic, optical and chemical properties. For example, many QDs display relatively strong emission in the visible region of the electromagnetic spectrum. Moreover, the wavelength of light absorbed and emitted is a function of the size of the QD. Because of their unique optical properties, QDs are promising materials for commercial applications as diverse as biological labeling, solar cells, catalysis, biological imaging, and light-emitting diodes to name just a few.
To date, the most studied and prepared of semiconductor materials have been the Group II-VI materials, namely, ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSe due to its tuneability over the visible region of the spectrum. Semiconductor nanoparticles are of academic and commercial interest due to their properties, which are unique from the properties of the corresponding crystalline bulk forms of the same semiconductor materials. Two fundamental factors, both related to the size of the individual nanoparticles, are responsible for their unique properties. The first is the large surface-to-volume ratio. As a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of small particles. The second factor is that, with semiconductor nanoparticles, there is a change in the electronic properties of the material with the size of the particle. Specifically, the band gap gradually becomes wider as the size of the particle decreases. This change in band gap is because of quantum confinement effects. This effect is a consequence of the confinement of an “electron in a box” giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, the “electron and hole” produced by the absorption of a photon are closer together than in the corresponding macrocrystalline material, resulting in non-negligible Coulombic interaction between the electron and hole. This leads to a narrow bandwidth emission that is dependent upon the particle size and composition. Consequently, quantum dots have higher kinetic energy than the corresponding macrocrystalline material and the first excitonic transition (band gap) increases in energy with decreasing particle diameter. Thus, quantum dots with a smaller diameter absorb and emit light of higher energy than do quantum dots with a larger diameter. In other words, the color of light absorbed and emitted can be “tuned” as a function of the particle diameter.
Single-core nanoparticles, which consist of a single semiconductor material (typically along with an outer organic passivating layer), tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface that lead to non-radiative electron-hole recombinations. One method of eliminating defects and dangling bonds is to grow a shell comprised of a second semiconductor material having a wider band-gap on the surface of the core particle to produce a “core-shell particle”. The shell semiconductor material preferably has a small lattice mismatch with the core material so that the interface between the two materials is minimized. Core-shell particles separate charge carriers confined in the core from surface states that would otherwise act as non-radiative recombination centers. A common example is ZnS grown on the surface of CdSe cores. Excessive strain can further result in defects and non-radiative electron-hole recombination resulting in low quantum efficiencies.
The coordination about the outer inorganic surface atoms is incomplete, with highly reactive “dangling bonds” on the surface, which can lead to particle agglomeration. This problem can be partially overcome by passivating (capping) the “bare” surface atoms with protecting organic groups. The capping or passivating of particles not only prevents particle agglomeration from occurring, it also protects the particle from its surrounding chemical environment and provides electronic stabilization (passivation) to the particles. The capping agent usually takes the form of a Lewis base compound covalently bound to surface metal atoms of the outer most inorganic layer of the particle.
Several synthetic methods for the preparation of semiconductor nanoparticles have been reported. Early routes applied conventional colloidal aqueous chemistry, while more recent methods involve the kinetically controlled precipitation of nanocrystallites, using organometallic compounds.
Since the optical properties of QDs are size-dependent, it is often desirable to produce populations of QDs with a high degree of monodispersity, i.e., with a high degree of uniformity in the size of the QDs in the population. Also, populations of QDs with high quantum yield (QY, the ratio of photons emitted to photons absorbed) are desirable. Methods have been reported to produce semiconductor QDs with high monodispersity and with quantum yields greater than 50%. Most of these methods are based on the original “nucleation and growth” method described by Murray, Norris and M. G. Bawendi, J. Am. Chem. Soc. 1993, 115, 8706, incorporated by reference in its entirety, but use other precursors than the organometallic ones used. Murray et al originally used organometallic solutions of metal-alkyls (R2M) M═Cd, Zn, Te; R═Me, Et and tri-n-octylphosphine sulfide/selenide (TOPS/Se) dissolved in tri-n-octylphosphine (TOP). These precursor solutions are injected into hot tri-n-octylphosphine oxide (TOPO) in the temperature range 120-400° C. depending on the material being produced. This produces TOPO-coated/capped semiconductor nanoparticles of Group II-VI material. The size of the particles is controlled by the temperature, concentration of precursor used and length of time at which the synthesis is undertaken, with larger particles being obtained at higher temperatures, higher precursor concentrations and prolonged reaction times. This organometallic route has advantages over other synthetic methods, including near monodispersity and high particle crystallinity.
Cadmium and other restricted heavy metals used in conventional QDs are highly toxic elements and represent a major concern in commercial applications. The inherent toxicity of cadmium-containing QDs precludes their use in applications involving animals or humans. For example recent studies suggest that QDs made of a cadmium chalcogenide semiconductor material can be cytotoxic in a biological environment unless protected. Specifically, oxidation or chemical attack through a variety of pathways can lead to the formation of cadmium ions on the QD surface that can be released into the surrounding environment. Although surface coatings such as ZnS can significantly reduce the toxicity, it may not completely eliminate it because QDs can be retained in cells or accumulated in the body for a long period of time, during which their coatings may undergo some form of degradation exposing the cadmium-rich core.
The toxicity affects not only the progress of biological applications but also other applications including optoelectronic and communication because heavy metal-based materials are widespread in many commercial products including household appliances such as IT and telecommunication equipment, lighting equipment, electrical and electronic tools, toys, leisure and sports equipment. A legislation to restrict or ban certain heavy metals in commercial products has been already implemented in many jurisdictions throughout the world. For example, European Union directive 2002/95/EC, known as the “Restrictions on the use of Hazardous Substances in electronic equipment” (or RoHS), bans the sale of new electrical and electronic equipment containing more than certain levels of lead, cadmium, mercury, hexavalent chromium along with polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants. This law requires manufacturers to find alternative materials and develop new engineering processes for the creation of common electronic equipment. In addition, on 1 Jun. 2007 a European Community Regulation came into force concerning chemicals and their safe use (EC 1907/2006). The Regulation concerns the Registration, Evaluation, Authorization and Restriction of Chemical substances and is known as “REACH”. The REACH Regulation imposes greater responsibility on industry to manage the risks associated with chemicals and to provide safety information on the substances. It is anticipated that similar regulations will be promulgated worldwide including China, Korea, Japan and the US. Thus, there are significant economic incentives to develop alternatives to Group II-VI QD materials.
Due to their increased covalent nature, Group III-V and Group IV-VI highly crystalline semiconductor nanoparticles are more difficult to prepare and much longer annealing times are usually required. However, there are now reports of Group III-VI and Group IV-VI materials being prepared in a similar manner to that used for the Group II-VI materials. Examples of such Group III-VI and Group IV-VI materials include GaN, GaP, GaAs, InP, InAs and for PbS and PbSe.
For all of the above methods, rapid particle nucleation followed by slow particle growth is essential for a narrow particle size distribution. All these synthetic methods are based on the original organometallic “nucleation and growth” method by Murray et al., which involves the rapid injection of the precursors into a hot solution of a Lewis base coordinating solvent (capping agent) which may also contain one of the precursors. The addition of the cooler solution subsequently lowers the reaction temperature and assists particle growth but inhibits further nucleation. The temperature is then maintained for a period of time, with the size of the resulting particles depending on reaction time, temperature and the ratio of capping agent to the precursor used. The resulting solution is cooled, followed by the addition of an excess of a polar solvent (methanol or ethanol or sometimes acetone) to produce a precipitate of the particles that can be isolated by filtration or centrifugation. Generally, larger particles precipitate more easily than smaller particles. Thus, precipitation provides a means of separating the quantum dots as a function of their size. Multiple precipitation steps are usually required to achieve narrow particle size distributions.
Fundamentally, these prior art preparations rely on the principle of particle nucleation followed by growth. Moreover, to have a mono-dispersed ensemble of nanoparticles there is preferably proper separation of nanoparticle nucleation from nanoparticle growth with the later occurring at a lower temperature from the former. This is achieved by rapid injection of one or both precursors into a hot coordinating solvent (containing the other precursor if otherwise not present), which initiates particle nucleation. The sudden addition of the cooler solution upon injection subsequently lowers the reaction temperature (the volume of solution added is typically about ⅓ of the total solution) and inhibits further nucleation maintaining a narrow nanoparticle size distribution. This method may work well for small-scale synthesis where one solution can be added rapidly to another while keeping a homogenous temperature throughout the reaction. However, on larger preparative scales, wherein large volumes of solution are required to be rapidly injected into one another, temperature differentials can occur within the reaction, which can lead to a large particle size distribution. Moreover, the need to perform multiple, size-selective, purification steps is not practical for the production of large quantities of QDs.
U.S. Pat. Nos. 7,588,828, 7,803,423, 7,985,446, and 8,062,703 (referred to herein collectively as “the seeding patents”) disclose synthetic methods for preparing monodisperse QD populations that do not rely on the hot injection methods and the size-selective purification steps described above. The disclosures of each of these patents are hereby incorporated by reference in their entireties. Briefly, the methods disclosed in these patents involve the use of a molecular cluster “seed” compound that serves as a template for the nucleation of the QD semiconductor material in solution. The cluster compound acts as a seed or nucleation point upon which nanoparticle growth can be initiated. In this way, a high temperature nucleation step is not necessary to initiate nanoparticle growth because suitable nucleation sites are already provided in the system by the molecular clusters. By providing nucleation sites that are more uniform than the nucleation sites employed in the methods described above, the synthesis provides a population of QDs that are essentially monodisperse. A significant advantage of the molecular seeding method is that it can be easily scaled-up.
The seeding methods described in the seeding patents can be used to make Group II-VI QDs and can also be used to make Group III-V and Group IV-VI QDs. But as mentioned above, the Group III-V and Group IV-VI QDs can be difficult to work with. Because of the high surface area-to-volume ratio, atoms on the surface of small QDs tend to diffuse from the surface of the smaller particles to the surface of the larger particles, a process known as Ostwald ripening. QDs of Group III-V materials such as InP and alloys thereof tend to be particularly unstable due to Ostwald ripening. Because Ostwald ripening decomposes the smaller QDs and promotes the growth of larger QDs, this process makes it difficult to obtain QDs that emit at shorter wavelengths, i.e., that emit in the blue and green regions of the visible spectrum.
There is thus a need in the art for improved synthetic methods of producing cadmium-free QDs that emit the shorter wavelengths of visible light.